butter v. margarine
Sunday, January 31, 2010 
Astronaut butter sculpture. Image courtesy of Flickr. Butter is made by agitating cream from cow’s milk until the fat aggregates. The remaining liquid, buttermilk, is then drained off. Butter is sometimes cultured, pasteurized, or salted.
Margarine was invented by Hippolyte Mège-Mouriés in 1869 as an inexpensive butter substitute. Mège-Mouriés used beef fat, but modern margarine is made from hydrogenated vegetable oil, skim milk, and emulsifiers. Color, flavor, and vitamins are often added to make it more like butter.
To protect the interests of the dairy industry in the 19th and 20th centuries, numerous laws were enacted that restricted or even prohibited the sale of margarine. Margarine had a few advantages over butter: it was cheaper, and could be kept at room temperature longer without spoiling. By the mid-20th century, margarine had become more popular than butter in Western diets.
Both spreads are water-in-oil emulsions containing about 80% fat, and can be used interchangeably in most recipes. It should be noted that margarine has less saturated fat than butter, and diets high in saturated fat have been linked to increased incidence of cardiovascular disease. However, when vegetable oils are hydrogenated to make margarine more solid at room temperatures, trans fatty acids are formed, which also may increase the risk for coronary heart disease.